Prosopagnosia

From Psy3241

Agnosia disorders include various cognitive disorders that deal with "not knowing" impariments

such as auditory agnosia which is the inability to recognize sounds or visual object agnosia

which is the inability to recognize familar objects that can be seen. Visual object agnosia

is similar to prosopagnosia and most patients with visual object agnosia also have

prosopagnosia. Prosopagnosia is the inability to recognize faces on sight and is usually

associated with bilateral occipital damage. A patient named Michael was diagnosed with

prosopagnosia and a few other agnosia disorders. Michael is incapable of recognizing any faces

desipte the familiarity and he has not been able to for 17 years. He never recognizes his mother

until she speaks and when asked to identify himself in family photos he is incapable of doing

so. He does have the ability to recollect faces if asked to describe someone famous or a family

member yet when presented with a picture of that person he still is not able to recognize their

face or to identify the person by picture.

The study of prosopagnosia has been crucial in the development of theories of face perception. Because prosopagnosia is not a unitary disorder (i.e., different people may show different types and levels of impairment) it has been argued that face perception involves a number of stages, each of which can be separately damaged. This is reflected not just in the amount of impairment displayed but also in the qualitative differences in impairment that a person with prosopagnosia may present with.

This sort of evidence has been crucial in supporting the theory that there may be a specific face perception system in the brain. This is counter-intuitive to many people as we do not experience faces as 'special' or perceived in a different way from the rest of the world.

One particularly interesting feature of prosopagnosia is that it suggests both a conscious and unconscious aspect to face recognition. Experiments have shown that when presented with a mixture of familiar and unfamiliar faces, people with prosopagnosia may be unable to successfully identify the people in the pictures, or even make a simple familiarity judgement ("this person seems familiar / unfamiliar"). However, when a measure of emotional response is taken (typically a measure of skin conductance) there tends to be an emotional response to familiar people even though no conscious recognition takes place.

This suggests emotion plays a significant role in face recognition, perhaps unsurprising when basic survival (particularly security) relies on identifying the people around you.

It is thought that Capgras delusion may be the reverse of prosopagnosia. In this condition people report conscious recognition of people from faces, but show no emotional response, perhaps leading to the delusional belief that their relative or spouse has been replaced by an impostor.